This invention relates to a novel catalyst for hydrogenating an olefinically unsaturated compound, which catalyst has a very high catalytic activity and is excellent in thermal resistance, storage stability and maintainability of catalytic activity and to a process for hydrogenating an olefinically unsaturated compound using the above hydrogenation catalyst.
Olefinically unsaturated polymers, representatives of which are conjugated diene polymers, have widely been used as elastomers and the like in industry. In these olefinically unsaturated polymers, however, their unsaturated bonds cause the deterioration of the weather resistance, thermal resistance and the like of the polymers though the unsaturated bonds can be utilized in vulcanization and the like. As a result, the unsaturated bonds bring about such a disadvantage that the application of the polymers is limited.
On the other hand, the weather resistance, thermal resistance and the like of the olefinically unsaturated polymer can be remarkably improved by hydrogenating the olefinically unsaturated bonds to convert the unsaturated polymer chains to saturated ones. As a hydrogenation catalyst to be used in the hydrogenation of the olefinically unsaturated polymers for the above purpose, there have heretofore been known heterogeneous system catalysts in which a metal such as nickel, platinum, palladium or the like has been supported on a carrier such as carbon, silica, alumina or the like and homogeneous system catalysts each composed of an organometallic compound whose metal is nickel, cobalt, titanium or the like and a reducing organometallic compound whose metal is aluminum, magnesium, lithium or the like.
The heterogeneous system hydrogenation catalyst is generally lower in catalytic activity than the homogeneous system hydrogenation catalyst and requires such sever conditions as high temperature and high pressure for sufficient hydrogenation reaction. Moreover, the hydrogenation reaction with the heterogenous system hydrogenation catalyst proceeds only when a compound to be hydrogenated is contacted with the catalyst; however, in the case of hydrogenating a polymer, the influence of the viscosity of a reaction system, the steric hindrance in the polymer chain and the like on the hydrogenation is greater than in the case of hydrogenating a low molecular weight compound, and hence, the contact of the polymer with the catalyst becomes difficult in some cases.
Accordingly, in order to hydrogenate olefinically unsaturated polymers with a great efficiency using a heterogeneous system hydrogenation catalyst, a large amount of the catalyst is required, which is uneconomical. At the same time, it is necessary to conduct the reaction at a higher temperature under higher pressure. Therefore, such problems are caused that the decomposition and gelation of polymer become easy to cause and the energy cost becomes high. In addition, when the hydrogenation conditions become severer, in the case of a copolymer of a conjugated diene with a vinyl aromatic compound such as styrene/butadiene rubber (SBR), there are such disadvantages that even the unsaturated bonds of the aromatic nucleus are hydrogenated and that it is difficult to selectively hydrogenate the conjugated diene portions.
On the other hand, the homogeneous system hydrogenation catalyst mentioned above is more advantageous than the heterogeneous hydrogenation catalyst in that the catalytic activity is generally higher, the amount of catalyst used is smaller and the reaction can be conducted under milder conditions. Moreover, when the hydrogenation conditions are selected adequately, even in the case of copolymerization of a conjugated diene with a vinyl aromatic compound, it becomes possible to selectively hydrogenate the conjugated diene portion.
However, in the case of the heterogeneous system hydrogenation catalyst, the activity is greatly varied depending upon the reduction state of the catalyst and hence there are such problems that the reproducibility is low and it is difficult to stably obtain a polymer having a high degree of hydrogenation. In addition, the catalyst constituents tend to be inactivated with the coexisting impurities and the like and this becomes a factor for the heterogeneous system hydrogenation catalyst to be poor in reproducibility. Moreover, it cannot be said that with the conventional homogeneous hydrogenation catalyst, the reaction velocity is sufficiently high and there is also such a problem that the reaction velocity is further lowered by the deterioration of catalytic activity resulting from impurities and reduction state of catalyst. In addition, these disadvantages have become a great obstacle to industrial utilization of the homogeneous hydrogenation catalyst.
Therefore, the fact is that a development has been strongly desired of a hydrogenation catalyst which has a high catalytic activity and is little affected by the co-existing impurities and with which a polymer having a high degree of hydrogenation can be stably obtained without being affected by the conditions for preparing the catalyst.
Now, several hydrogenation catalysts have been known in which a bis(cyclopentadienyl or its derivative) compound of a transition metal is used as one of the constituents, and, for example, M. F. Sloan et al., J. Am. Chem. Soc., 85, 4014-4018 (1965); Y. Tajima et al., J. Org. Chem., 33, 1689-1690 (1968); U.S. Pat. No. 4,501,857; JP-A-61 (1986)-28,507; JP-A-1 (1989)-275,605; JP-A-2 (1990)-51,503; U.S. Pat. No. 5,583,185; and the like propose a system composed of bis(cyclopentadienyl)titanium dichloride and an organoaluminum compound or organiolithium compound; a system composed of a bis(cyclopentadienyl)titanium dialkyl and an organolithium compound; a system composed of bis(cyclopentadienyl)titanium diphenyl and an organolithium compound; a system composed of bis(cyclopentadienyl)titanium dibenzyl and an organometallic compound whose metal is of Groups I to III of the Periodic Table; a system composed of a bis(cyclopentadienyl)titanium di(alkoxyphenyl) or bis(cyclopentadienyl)titanium bis(diphenylphosphinomethylene) and an organolithium compound, organomagnesium compound or organoaluminum compound; and the like. However, these hydrogenation catalysts cannot satisfy sufficiently the above-mentioned desire.
Furthermore, U.S. Pat. No. 4,501,857 proposes a system composed of a bis(cyclopentadienyl)titanium dialkoxy or a bis(cyclopentadienyl)titanium diaryloxy and an organolithium compound, too. However, with these catalysts, the number of carbon atoms of the alkoxy group or aryloxy group bonded to the titanium atom is limited to 8 or less, and in particular, a development has been desired of a novel homogeneous system hydrogenation catalyst having a further improved catalytic activity.
Moreover, U.S. Pat. No. 5,270,274; JP-A-6-220,122; JP-A-6-220,123 and the like propose hydrogenation catalysts comprising substantially an aluminum component. However, aluminum-free catalyst systems have been desired in industry. Incidentally, U.S. Pat. No. 4,980,421 also describes examples in which a catalyst system containing no aluminum component [a system composed of a steric hindrance phenoxylithium compound and bis(cyclopentadienyl)titanium dichloride] is used, but in the case of this catalyst system, as shown in Comparative Example 6 described hereinafter, the catalytic activity cannot be said to be always sufficient. In addition, the catalyst tends to be deactivated at a high temperature not lower than 90.degree. C. and hence has an industrial problem. Therefore, a development has been also desired of a hydrogenation catalyst which is free of aluminum but has a high catalytic activity and is not deactivated even under such high temperature conditions.